1. The Field of the Invention
The present invention is directed to an improved Thermal DeNO.sub.x process for control of NO.sub.x emissions. More particularly, the present invention is directed to methods for controlling NO.sub.x emissions from stationary combustion systems such as power plant boilers, process furnaces, and incinerators.
2. Technology Review
The problems of waste management in the United States urgently require the development of an environmentally acceptable incineration technology, but for one important class of pollutants, nitrogen oxides (commonly referred to as "NO.sub.x "), the presently available NO.sub.x control technology provides only a very limited degree of control.
A survey by the United States Environmental Protection Agency indicates that the United States generates about 140 million metric tons of industrial waste and 230 million metric tons of municipal waste annually. In the past, most waste was disposed of by landfill, but such approaches are inherently unsatisfactory because the toxic materials in the waste are not destroyed or rendered innocuous but are merely isolated. Recognition of the dangers inherent in disposal by isolation has lead to increasingly tighter control and monitoring of these disposal practices, making them impractical and prohibitively expensive. It is not uncommon for landfills to be closed for reasons of environmental safety even when there are not alternative disposal methods available.
Incineration is potentially the ideal solution to the problem of waste management since toxic organic materials can be completely destroyed and most of the toxic inorganic materials of concern can be converted to an inert glass by operation at temperatures above the ash fusion point. Most of the problems which have given incinerators a poor reputation in the past have satisfactory answers. For instance, emissions of acid gases such as SO.sub.2 and HCl can be controlled by wet scrubbers.
However, control of NO.sub.x emissions from incinerators is a problem to which no presently available technology provides a fully satisfactory answer. While the amount of NO.sub.x produced by burning waste can be minimized by managing the combustion process, waste typically contains substantial amounts of chemically bound nitrogen such that NO.sub.x levels are usually unacceptably high, even with careful control of the combustion process. As a result, some form of post combustion NO.sub.x control technology must be used in incineration processes.
Two types of post combustion NO.sub.x control technologies are presently available, selective catalytic reduction (SCR) and selective noncatalytic reduction (i.e., Thermal DeNO.sub.x). Applications of SCR to incinerators are generally regarded as nonfeasible because waste contains virtually all possible trace impurities and these impurities can act as catalyst poisons.
Because no better technology currently exits, the Thermal DeNO.sub.x process has been accepted as the best available NO.sub.x control technology for incinerators. In the Thermal DeNO.sub.x process, NH.sub.3 is contacted with flue gas at a temperature in the range from 900.degree. C to 950.degree. C. A homogeneous gas phase reaction occurs which reduces the NO in the flue gas to molecular nitrogen (N.sub.2) and water (H.sub.2 O). The performance of Thermal DeNO.sub.x in actual incinerator applications has, however, been highly disappointing.
In most applications, the performance of the Thermal DeNO.sub.x process depends primarily on the available reaction time, i.e., the length of time the flue gas spends in the temperature range suitable for Thermal DeNO.sub.x. For applications in which the available reaction time is less than 0.2 seconds, Thermal DeNO.sub.x typically achieves NO.sub.x reductions in the 60% to 80% range. For applications in which the available reaction time is greater than 0.2 seconds NO.sub.x reductions in the 80% to 90% range have commonly been achieved.
The design of a modern incinerator provides the post-flame gases with a residence time generally greater than 1.0 seconds in the temperature range appropriate to Thermal DeNO.sub.x. Hence, one might expect incinerators to be a very favorable application for Thermal DeNO.sub.x. Instead, however, NO.sub.x reduction in incinerators is actually 40% or less.
The poor performance of Thermal DeNO.sub.x on incinerators is, in part, a result of the fact that the temperature of the flue gas in incinerators is more highly variable than it is in other combustion systems. Waste is inherently a fuel with a highly variable BTU content. This variability causes the temperature of the flue gases downstream of the combustion zone to be nonhomogeneous in space and to fluctuate in time. If the temperature of the flue gas is a little too low at the point where the NH.sub.3 is injected, slight or no NO reduction occurs. If the temperature is too high, the NH.sub.3 has some tendency to oxidize to produce NO, and the net reduction of NO is poor or more NO may even be produced.
While there is a narrow range of temperatures in which nearly quantitative NO reductions are possible, at temperature below this range the reaction time between the NO and NH.sub.3 is too slow to be useful while at temperature above this range the NH.sub.3 oxidizes to form NO. Because this "temperature window" for the Thermal DeNO.sub.x process is narrow, successful application of the process is always critically dependent on locating the ammonia injection system at the location at which the average temperature is optimum for the process. In any application, however, the temperature will be nonhomogeneous, and process performance will be determined by an average over a temperature range. Since this always includes some temperatures which are too high and some which are too low for good NO.sub.x reduction, the practical extent of NO.sub.x control which the process can provide is always significantly less than is achieved in laboratory experiments.
Since the width of the Thermal DeNO.sub.x temperature window increases with increasing reaction time, the longer reaction time available in incinerators compensates, in part, for this difficulty. However, there is an additional problem: the optimum temperature for the Thermal DeNO.sub.x process may be shifted. For example, as shown in FIG. 1, (quoted from R. K. Lyon and J. E. Hardy, "Discovery and Development of the Thermal DeNO.sub.x Process," Ind. Eng. Chem. Fundam. Vol. 25, page 19, 1986; see also Environmental Science and Technology, Vol. 21, page 232, 1987) hydrogen (H.sub.2) mixed with the ammonia shifts the Thermal DeNO.sub.x temperature window to lower temperatures. The magnitude of the temperature shift increases as the amount of H.sub.2 is increased. This shifting of the temperature window is a general effect which occurs with other combustible materials, including CO, natural gas, etc.
This is a problem because waste material burned in an incinerator is far less homogeneous than any conventional fuel. Consequently, the mixing of fuel and air in an incinerator is much less intimate than with conventional fuel. Some regions in an incinerator's primary combustion zone are strongly fuel rich and produce a flue gas containing substantial amounts of CO and lesser amounts of other combustibles.
Modern incinerators are designed so that the gases leaving the primary combustion region have an extended residence time at high temperature. This permits flue gas coming from regions of fuel-rich combustion to mix with flue gas from fuel-lean regions, i.e., flue gas containing CO and no O.sub.2 mixes with flue gas containing O.sub.2 and virtually no CO. Because CO oxidation at these temperatures is a very rapid reaction, the overall rate of CO removal is almost entirely mixing limited.
In the Thermal DeNO.sub.x process, gaseous ammonia is mixed with a carrier gas such as steam or compressed air and injected via nozzles into the hot flue gas. These ammonia-containing jets cause intense local mixing. Since the portions of the flue gas which contain CO are not uniformly distributed, this intense local mixing creates some regions in which O.sub.2 and CO are both present and some regions in which O.sub.2 is present and CO is virtually absent. If the Thermal DeNO.sub.x process is applied to an incinerator, some of the flue gas being treated will contain significant amounts of CO and some will not.
The extent to which variable amounts of CO causes difficulties for the Thermal DeNO.sub.x process has been examined by computer modeling. In that study, it was assumed that the average CO concentration in the flue gas undergoing the Thermal DeNO.sub.x process was 500 ppm, and that this average could be roughly approximated by considering regions of flue gas in which the CO concentration was 0 ppm and 1000 ppm.
The results of these modeling calculations, shown graphically in FIG. 2, demonstrate that NO reductions in excess of 50% are possible over a substantial temperature range in the absence of CO and over a substantial temperature range in the presence of 1000 ppm CO. Unfortunately, these ranges have very little overlap. Thus, the presence of variable amounts of CO can substantially degrade the performance of the Thermal DeNO.sub.x process in much the same way as do variations in the flue gas temperature.
From the foregoing, it is apparent that what is currently needed in the art are methods for controlling NO.sub.x emissions from stationary combustion systems having variable flue gas temperatures. It would also be an advancement in the art to provide methods for controlling NO.sub.x emissions from stationary combustion systems having variable amounts of CO in the combustion effluents.
Such methods are disclosed and claimed herein.